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  <div class="section" id="command-line-and-environment">
<span id="using-on-general"></span><h1>1. Command line and environment<a class="headerlink" href="#command-line-and-environment" title="Permalink to this headline">¶</a></h1>
<p>The CPython interpreter scans the command line and the environment for various
settings.</p>
<div class="impl-detail compound">
<p><strong>CPython implementation detail:</strong> Other implementations’ command line schemes may differ.  See
<a class="reference internal" href="../reference/introduction.html#implementations"><span class="std std-ref">Alternate Implementations</span></a> for further resources.</p>
</div>
<div class="section" id="command-line">
<span id="using-on-cmdline"></span><h2>1.1. Command line<a class="headerlink" href="#command-line" title="Permalink to this headline">¶</a></h2>
<p>When invoking Python, you may specify any of these options:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>python <span class="o">[</span>-bBdEhiIOqsSuvVWx?<span class="o">]</span> <span class="o">[</span>-c <span class="nb">command</span> <span class="p">|</span> -m module-name <span class="p">|</span> script <span class="p">|</span> - <span class="o">]</span> <span class="o">[</span>args<span class="o">]</span>
</pre></div>
</div>
<p>The most common use case is, of course, a simple invocation of a script:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>python myscript.py
</pre></div>
</div>
<div class="section" id="interface-options">
<span id="using-on-interface-options"></span><h3>1.1.1. Interface options<a class="headerlink" href="#interface-options" title="Permalink to this headline">¶</a></h3>
<p>The interpreter interface resembles that of the UNIX shell, but provides some
additional methods of invocation:</p>
<ul class="simple">
<li>When called with standard input connected to a tty device, it prompts for
commands and executes them until an EOF (an end-of-file character, you can
produce that with <code class="kbd docutils literal"><span class="pre">Ctrl-D</span></code> on UNIX or <code class="kbd docutils literal"><span class="pre">Ctrl-Z,</span> <span class="pre">Enter</span></code> on Windows) is read.</li>
<li>When called with a file name argument or with a file as standard input, it
reads and executes a script from that file.</li>
<li>When called with a directory name argument, it reads and executes an
appropriately named script from that directory.</li>
<li>When called with <code class="docutils literal"><span class="pre">-c</span> <span class="pre">command</span></code>, it executes the Python statement(s) given as
<em>command</em>.  Here <em>command</em> may contain multiple statements separated by
newlines. Leading whitespace is significant in Python statements!</li>
<li>When called with <code class="docutils literal"><span class="pre">-m</span> <span class="pre">module-name</span></code>, the given module is located on the
Python module path and executed as a script.</li>
</ul>
<p>In non-interactive mode, the entire input is parsed before it is executed.</p>
<p>An interface option terminates the list of options consumed by the interpreter,
all consecutive arguments will end up in <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal"><span class="pre">sys.argv</span></code></a> – note that the first
element, subscript zero (<code class="docutils literal"><span class="pre">sys.argv[0]</span></code>), is a string reflecting the program’s
source.</p>
<dl class="cmdoption">
<dt id="cmdoption-c">
<code class="descname">-c</code><code class="descclassname"> &lt;command&gt;</code><a class="headerlink" href="#cmdoption-c" title="Permalink to this definition">¶</a></dt>
<dd><p>Execute the Python code in <em>command</em>.  <em>command</em> can be one or more
statements separated by newlines, with significant leading whitespace as in
normal module code.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal"><span class="pre">sys.argv</span></code></a> will be
<code class="docutils literal"><span class="pre">&quot;-c&quot;</span></code> and the current directory will be added to the start of
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> (allowing modules in that directory to be imported as top
level modules).</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-m">
<code class="descname">-m</code><code class="descclassname"> &lt;module-name&gt;</code><a class="headerlink" href="#cmdoption-m" title="Permalink to this definition">¶</a></dt>
<dd><p>Search <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> for the named module and execute its contents as
the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal"><span class="pre">__main__</span></code></a> module.</p>
<p>Since the argument is a <em>module</em> name, you must not give a file extension
(<code class="docutils literal"><span class="pre">.py</span></code>).  The module name should be a valid absolute Python module name, but
the implementation may not always enforce this (e.g. it may allow you to
use a name that includes a hyphen).</p>
<p>Package names (including namespace packages) are also permitted. When a
package name is supplied instead
of a normal module, the interpreter will execute <code class="docutils literal"><span class="pre">&lt;pkg&gt;.__main__</span></code> as
the main module. This behaviour is deliberately similar to the handling
of directories and zipfiles that are passed to the interpreter as the
script argument.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This option cannot be used with built-in modules and extension modules
written in C, since they do not have Python module files. However, it
can still be used for precompiled modules, even if the original source
file is not available.</p>
</div>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal"><span class="pre">sys.argv</span></code></a> will be the
full path to the module file (while the module file is being located, the
first element will be set to <code class="docutils literal"><span class="pre">&quot;-m&quot;</span></code>). As with the <a class="reference internal" href="#cmdoption-c"><code class="xref std std-option docutils literal"><span class="pre">-c</span></code></a> option,
the current directory will be added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.</p>
<p>Many standard library modules contain code that is invoked on their execution
as a script.  An example is the <a class="reference internal" href="../library/timeit.html#module-timeit" title="timeit: Measure the execution time of small code snippets."><code class="xref py py-mod docutils literal"><span class="pre">timeit</span></code></a> module:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>python -mtimeit -s <span class="s1">&#39;setup here&#39;</span> <span class="s1">&#39;benchmarked code here&#39;</span>
python -mtimeit -h <span class="c1"># for details</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="docutils">
<dt><a class="reference internal" href="../library/runpy.html#runpy.run_module" title="runpy.run_module"><code class="xref py py-func docutils literal"><span class="pre">runpy.run_module()</span></code></a></dt>
<dd>Equivalent functionality directly available to Python code</dd>
</dl>
<p class="last"><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0338"><strong>PEP 338</strong></a> – Executing modules as scripts</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.1: </span>Supply the package name to run a <code class="docutils literal"><span class="pre">__main__</span></code> submodule.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.4: </span>namespace packages are also supported</p>
</div>
</dd></dl>

<dl class="describe">
<dt>
<code class="descname">-</code></dt>
<dd><p>Read commands from standard input (<a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal"><span class="pre">sys.stdin</span></code></a>).  If standard input is
a terminal, <a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal"><span class="pre">-i</span></code></a> is implied.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal"><span class="pre">sys.argv</span></code></a> will be
<code class="docutils literal"><span class="pre">&quot;-&quot;</span></code> and the current directory will be added to the start of
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.</p>
</dd></dl>

<dl class="describe">
<dt>
<code class="descname">&lt;script&gt;</code></dt>
<dd><p>Execute the Python code contained in <em>script</em>, which must be a filesystem
path (absolute or relative) referring to either a Python file, a directory
containing a <code class="docutils literal"><span class="pre">__main__.py</span></code> file, or a zipfile containing a
<code class="docutils literal"><span class="pre">__main__.py</span></code> file.</p>
<p>If this option is given, the first element of <a class="reference internal" href="../library/sys.html#sys.argv" title="sys.argv"><code class="xref py py-data docutils literal"><span class="pre">sys.argv</span></code></a> will be the
script name as given on the command line.</p>
<p>If the script name refers directly to a Python file, the directory
containing that file is added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>, and the
file is executed as the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal"><span class="pre">__main__</span></code></a> module.</p>
<p>If the script name refers to a directory or zipfile, the script name is
added to the start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> and the <code class="docutils literal"><span class="pre">__main__.py</span></code> file in
that location is executed as the <a class="reference internal" href="../library/__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal"><span class="pre">__main__</span></code></a> module.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="../library/runpy.html#runpy.run_path" title="runpy.run_path"><code class="xref py py-func docutils literal"><span class="pre">runpy.run_path()</span></code></a></dt>
<dd>Equivalent functionality directly available to Python code</dd>
</dl>
</div>
</dd></dl>

<p>If no interface option is given, <a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal"><span class="pre">-i</span></code></a> is implied, <code class="docutils literal"><span class="pre">sys.argv[0]</span></code> is
an empty string (<code class="docutils literal"><span class="pre">&quot;&quot;</span></code>) and the current directory will be added to the
start of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.  Also, tab-completion and history editing is
automatically enabled, if available on your platform (see
<a class="reference internal" href="../library/site.html#rlcompleter-config"><span class="std std-ref">Readline configuration</span></a>).</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="../tutorial/interpreter.html#tut-invoking"><span class="std std-ref">Invoking the Interpreter</span></a></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.4: </span>Automatic enabling of tab-completion and history editing.</p>
</div>
</div>
<div class="section" id="generic-options">
<h3>1.1.2. Generic options<a class="headerlink" href="#generic-options" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption">
<code class="descname">-?</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-h">
<code class="descname">-h</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-h" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-help">
<code class="descname">--help</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-help" title="Permalink to this definition">¶</a></dt>
<dd><p>Print a short description of all command line options.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-v">
<code class="descname">-V</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-v" title="Permalink to this definition">¶</a></dt>
<dt id="cmdoption-version">
<code class="descname">--version</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-version" title="Permalink to this definition">¶</a></dt>
<dd><p>Print the Python version number and exit.  Example output could be:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>Python <span class="m">3</span>.6.0b2+
</pre></div>
</div>
<p>When given twice, print more information about the build, like:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>Python <span class="m">3</span>.6.0b2+ <span class="o">(</span><span class="m">3</span>.6:84a3c5003510+, Oct <span class="m">26</span> <span class="m">2016</span>, <span class="m">02</span>:33:55<span class="o">)</span>
<span class="o">[</span>GCC <span class="m">6</span>.2.0 <span class="m">20161005</span><span class="o">]</span>
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.6: </span>The <code class="docutils literal"><span class="pre">-VV</span></code> option.</p>
</div>
</dd></dl>

</div>
<div class="section" id="miscellaneous-options">
<span id="using-on-misc-options"></span><h3>1.1.3. Miscellaneous options<a class="headerlink" href="#miscellaneous-options" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption-b">
<code class="descname">-b</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-b" title="Permalink to this definition">¶</a></dt>
<dd><p>Issue a warning when comparing <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#bytearray" title="bytearray"><code class="xref py py-class docutils literal"><span class="pre">bytearray</span></code></a> with
<a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal"><span class="pre">str</span></code></a> or <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a> with <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal"><span class="pre">int</span></code></a>.  Issue an error when the
option is given twice (<code class="xref std std-option docutils literal"><span class="pre">-bb</span></code>).</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.5: </span>Affects comparisons of <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal"><span class="pre">bytes</span></code></a> with <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal"><span class="pre">int</span></code></a>.</p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="id1">
<code class="descname">-B</code><code class="descclassname"></code><a class="headerlink" href="#id1" title="Permalink to this definition">¶</a></dt>
<dd><p>If given, Python won’t try to write <code class="docutils literal"><span class="pre">.pyc</span></code> files on the
import of source modules.  See also <span class="target" id="index-1"></span><a class="reference internal" href="#envvar-PYTHONDONTWRITEBYTECODE"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONDONTWRITEBYTECODE</span></code></a>.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-d">
<code class="descname">-d</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-d" title="Permalink to this definition">¶</a></dt>
<dd><p>Turn on parser debugging output (for wizards only, depending on compilation
options).  See also <span class="target" id="index-2"></span><a class="reference internal" href="#envvar-PYTHONDEBUG"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONDEBUG</span></code></a>.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-e">
<code class="descname">-E</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-e" title="Permalink to this definition">¶</a></dt>
<dd><p>Ignore all <span class="target" id="index-3"></span><code class="xref std std-envvar docutils literal"><span class="pre">PYTHON*</span></code> environment variables, e.g.
<span class="target" id="index-4"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a> and <span class="target" id="index-5"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHOME</span></code></a>, that might be set.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-i">
<code class="descname">-i</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-i" title="Permalink to this definition">¶</a></dt>
<dd><p>When a script is passed as first argument or the <a class="reference internal" href="#cmdoption-c"><code class="xref std std-option docutils literal"><span class="pre">-c</span></code></a> option is used,
enter interactive mode after executing the script or the command, even when
<a class="reference internal" href="../library/sys.html#sys.stdin" title="sys.stdin"><code class="xref py py-data docutils literal"><span class="pre">sys.stdin</span></code></a> does not appear to be a terminal.  The
<span class="target" id="index-6"></span><a class="reference internal" href="#envvar-PYTHONSTARTUP"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONSTARTUP</span></code></a> file is not read.</p>
<p>This can be useful to inspect global variables or a stack trace when a script
raises an exception.  See also <span class="target" id="index-7"></span><a class="reference internal" href="#envvar-PYTHONINSPECT"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONINSPECT</span></code></a>.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="id2">
<code class="descname">-I</code><code class="descclassname"></code><a class="headerlink" href="#id2" title="Permalink to this definition">¶</a></dt>
<dd><p>Run Python in isolated mode. This also implies -E and -s.
In isolated mode <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> contains neither the script’s directory nor
the user’s site-packages directory. All <span class="target" id="index-8"></span><code class="xref std std-envvar docutils literal"><span class="pre">PYTHON*</span></code> environment
variables are ignored, too. Further restrictions may be imposed to prevent
the user from injecting malicious code.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-o">
<code class="descname">-O</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-o" title="Permalink to this definition">¶</a></dt>
<dd><p>Turn on basic optimizations.  See also <span class="target" id="index-9"></span><a class="reference internal" href="#envvar-PYTHONOPTIMIZE"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONOPTIMIZE</span></code></a>.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-oo">
<code class="descname">-OO</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-oo" title="Permalink to this definition">¶</a></dt>
<dd><p>Discard docstrings in addition to the <a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal"><span class="pre">-O</span></code></a> optimizations.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-q">
<code class="descname">-q</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-q" title="Permalink to this definition">¶</a></dt>
<dd><p>Don’t display the copyright and version messages even in interactive mode.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.2.</span></p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-r">
<code class="descname">-R</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-r" title="Permalink to this definition">¶</a></dt>
<dd><p>Kept for compatibility.  On Python 3.3 and greater, hash randomization is
turned on by default.</p>
<p>On previous versions of Python, this option turns on hash randomization,
so that the <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal"><span class="pre">__hash__()</span></code></a> values of str, bytes and datetime
are “salted” with an unpredictable random value.  Although they remain
constant within an individual Python process, they are not predictable
between repeated invocations of Python.</p>
<p>Hash randomization is intended to provide protection against a
denial-of-service caused by carefully-chosen inputs that exploit the worst
case performance of a dict construction, O(n^2) complexity.  See
<a class="reference external" href="http://www.ocert.org/advisories/ocert-2011-003.html">http://www.ocert.org/advisories/ocert-2011-003.html</a> for details.</p>
<p><span class="target" id="index-10"></span><a class="reference internal" href="#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHASHSEED</span></code></a> allows you to set a fixed value for the hash
seed secret.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.2.3.</span></p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-s">
<code class="descname">-s</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-s" title="Permalink to this definition">¶</a></dt>
<dd><p>Don’t add the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a> to
<a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><span class="target" id="index-11"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="id3">
<code class="descname">-S</code><code class="descclassname"></code><a class="headerlink" href="#id3" title="Permalink to this definition">¶</a></dt>
<dd><p>Disable the import of the module <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal"><span class="pre">site</span></code></a> and the site-dependent
manipulations of <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a> that it entails.  Also disable these
manipulations if <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal"><span class="pre">site</span></code></a> is explicitly imported later (call
<a class="reference internal" href="../library/site.html#site.main" title="site.main"><code class="xref py py-func docutils literal"><span class="pre">site.main()</span></code></a> if you want them to be triggered).</p>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-u">
<code class="descname">-u</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-u" title="Permalink to this definition">¶</a></dt>
<dd><p>Force the binary layer of the stdout and stderr streams (which is
available as their <code class="docutils literal"><span class="pre">buffer</span></code> attribute) to be unbuffered. The text I/O
layer will still be line-buffered if writing to the console, or
block-buffered if redirected to a non-interactive file.</p>
<p>See also <span class="target" id="index-12"></span><a class="reference internal" href="#envvar-PYTHONUNBUFFERED"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONUNBUFFERED</span></code></a>.</p>
</dd></dl>

<dl class="cmdoption">
<dt id="id4">
<code class="descname">-v</code><code class="descclassname"></code><a class="headerlink" href="#id4" title="Permalink to this definition">¶</a></dt>
<dd><p>Print a message each time a module is initialized, showing the place
(filename or built-in module) from which it is loaded.  When given twice
(<code class="xref std std-option docutils literal"><span class="pre">-vv</span></code>), print a message for each file that is checked for when
searching for a module.  Also provides information on module cleanup at exit.
See also <span class="target" id="index-13"></span><a class="reference internal" href="#envvar-PYTHONVERBOSE"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONVERBOSE</span></code></a>.</p>
</dd></dl>

<span class="target" id="using-on-warnings"></span><dl class="cmdoption">
<dt id="cmdoption-w">
<code class="descname">-W</code><code class="descclassname"> arg</code><a class="headerlink" href="#cmdoption-w" title="Permalink to this definition">¶</a></dt>
<dd><p>Warning control.  Python’s warning machinery by default prints warning
messages to <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal"><span class="pre">sys.stderr</span></code></a>.  A typical warning message has the following
form:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>file:line: category: message
</pre></div>
</div>
<p>By default, each warning is printed once for each source line where it
occurs.  This option controls how often warnings are printed.</p>
<p>Multiple <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal"><span class="pre">-W</span></code></a> options may be given; when a warning matches more than
one option, the action for the last matching option is performed.  Invalid
<a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal"><span class="pre">-W</span></code></a> options are ignored (though, a warning message is printed about
invalid options when the first warning is issued).</p>
<p>Warnings can also be controlled from within a Python program using the
<a class="reference internal" href="../library/warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal"><span class="pre">warnings</span></code></a> module.</p>
<p>The simplest form of argument is one of the following action strings (or a
unique abbreviation):</p>
<dl class="docutils">
<dt><code class="docutils literal"><span class="pre">ignore</span></code></dt>
<dd>Ignore all warnings.</dd>
<dt><code class="docutils literal"><span class="pre">default</span></code></dt>
<dd>Explicitly request the default behavior (printing each warning once per
source line).</dd>
<dt><code class="docutils literal"><span class="pre">all</span></code></dt>
<dd>Print a warning each time it occurs (this may generate many messages if a
warning is triggered repeatedly for the same source line, such as inside a
loop).</dd>
<dt><code class="docutils literal"><span class="pre">module</span></code></dt>
<dd>Print each warning only the first time it occurs in each module.</dd>
<dt><code class="docutils literal"><span class="pre">once</span></code></dt>
<dd>Print each warning only the first time it occurs in the program.</dd>
<dt><code class="docutils literal"><span class="pre">error</span></code></dt>
<dd>Raise an exception instead of printing a warning message.</dd>
</dl>
<p>The full form of argument is:</p>
<div class="highlight-sh"><div class="highlight"><pre><span></span>action:message:category:module:line
</pre></div>
</div>
<p>Here, <em>action</em> is as explained above but only applies to messages that match
the remaining fields.  Empty fields match all values; trailing empty fields
may be omitted.  The <em>message</em> field matches the start of the warning message
printed; this match is case-insensitive.  The <em>category</em> field matches the
warning category.  This must be a class name; the match tests whether the
actual warning category of the message is a subclass of the specified warning
category.  The full class name must be given.  The <em>module</em> field matches the
(fully-qualified) module name; this match is case-sensitive.  The <em>line</em>
field matches the line number, where zero matches all line numbers and is
thus equivalent to an omitted line number.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p><a class="reference internal" href="../library/warnings.html#module-warnings" title="warnings: Issue warning messages and control their disposition."><code class="xref py py-mod docutils literal"><span class="pre">warnings</span></code></a> – the warnings module</p>
<p><span class="target" id="index-14"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0230"><strong>PEP 230</strong></a> – Warning framework</p>
<p class="last"><span class="target" id="index-15"></span><a class="reference internal" href="#envvar-PYTHONWARNINGS"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONWARNINGS</span></code></a></p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="cmdoption-x">
<code class="descname">-x</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-x" title="Permalink to this definition">¶</a></dt>
<dd><p>Skip the first line of the source, allowing use of non-Unix forms of
<code class="docutils literal"><span class="pre">#!cmd</span></code>.  This is intended for a DOS specific hack only.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The line numbers in error messages will be off by one.</p>
</div>
</dd></dl>

<dl class="cmdoption">
<dt id="id5">
<code class="descname">-X</code><code class="descclassname"></code><a class="headerlink" href="#id5" title="Permalink to this definition">¶</a></dt>
<dd><p>Reserved for various implementation-specific options.  CPython currently
defines the following possible values:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">-X</span> <span class="pre">faulthandler</span></code> to enable <a class="reference internal" href="../library/faulthandler.html#module-faulthandler" title="faulthandler: Dump the Python traceback."><code class="xref py py-mod docutils literal"><span class="pre">faulthandler</span></code></a>;</li>
<li><code class="docutils literal"><span class="pre">-X</span> <span class="pre">showrefcount</span></code> to output the total reference count and number of used
memory blocks when the program finishes or after each statement in the
interactive interpreter. This only works on debug builds.</li>
<li><code class="docutils literal"><span class="pre">-X</span> <span class="pre">tracemalloc</span></code> to start tracing Python memory allocations using the
<a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal"><span class="pre">tracemalloc</span></code></a> module. By default, only the most recent frame is
stored in a traceback of a trace. Use <code class="docutils literal"><span class="pre">-X</span> <span class="pre">tracemalloc=NFRAME</span></code> to start
tracing with a traceback limit of <em>NFRAME</em> frames. See the
<a class="reference internal" href="../library/tracemalloc.html#tracemalloc.start" title="tracemalloc.start"><code class="xref py py-func docutils literal"><span class="pre">tracemalloc.start()</span></code></a> for more information.</li>
<li><code class="docutils literal"><span class="pre">-X</span> <span class="pre">showalloccount</span></code> to output the total count of allocated objects for
each type when the program finishes. This only works when Python was built with
<code class="docutils literal"><span class="pre">COUNT_ALLOCS</span></code> defined.</li>
</ul>
<p>It also allows passing arbitrary values and retrieving them through the
<a class="reference internal" href="../library/sys.html#sys._xoptions" title="sys._xoptions"><code class="xref py py-data docutils literal"><span class="pre">sys._xoptions</span></code></a> dictionary.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.2: </span>The <a class="reference internal" href="#id5"><code class="xref std std-option docutils literal"><span class="pre">-X</span></code></a> option was added.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.3: </span>The <code class="docutils literal"><span class="pre">-X</span> <span class="pre">faulthandler</span></code> option.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4: </span>The <code class="docutils literal"><span class="pre">-X</span> <span class="pre">showrefcount</span></code> and <code class="docutils literal"><span class="pre">-X</span> <span class="pre">tracemalloc</span></code> options.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.6: </span>The <code class="docutils literal"><span class="pre">-X</span> <span class="pre">showalloccount</span></code> option.</p>
</div>
</dd></dl>

</div>
<div class="section" id="options-you-shouldn-t-use">
<h3>1.1.4. Options you shouldn’t use<a class="headerlink" href="#options-you-shouldn-t-use" title="Permalink to this headline">¶</a></h3>
<dl class="cmdoption">
<dt id="cmdoption-j">
<code class="descname">-J</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-j" title="Permalink to this definition">¶</a></dt>
<dd><p>Reserved for use by <a class="reference external" href="http://www.jython.org/">Jython</a>.</p>
</dd></dl>

</div>
</div>
<div class="section" id="environment-variables">
<span id="using-on-envvars"></span><h2>1.2. Environment variables<a class="headerlink" href="#environment-variables" title="Permalink to this headline">¶</a></h2>
<p>These environment variables influence Python’s behavior, they are processed
before the command-line switches other than -E or -I.  It is customary that
command-line switches override environmental variables where there is a
conflict.</p>
<dl class="envvar">
<dt id="envvar-PYTHONHOME">
<code class="descname">PYTHONHOME</code><a class="headerlink" href="#envvar-PYTHONHOME" title="Permalink to this definition">¶</a></dt>
<dd><p>Change the location of the standard Python libraries.  By default, the
libraries are searched in <code class="file docutils literal"><em><span class="pre">prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code> and
<code class="file docutils literal"><em><span class="pre">exec_prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code>, where <code class="file docutils literal"><em><span class="pre">prefix</span></em></code> and
<code class="file docutils literal"><em><span class="pre">exec_prefix</span></em></code> are installation-dependent directories, both defaulting
to <code class="file docutils literal"><span class="pre">/usr/local</span></code>.</p>
<p>When <span class="target" id="index-16"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHOME</span></code></a> is set to a single directory, its value replaces
both <code class="file docutils literal"><em><span class="pre">prefix</span></em></code> and <code class="file docutils literal"><em><span class="pre">exec_prefix</span></em></code>.  To specify different values
for these, set <span class="target" id="index-17"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHOME</span></code></a> to <code class="file docutils literal"><em><span class="pre">prefix</span></em><span class="pre">:</span><em><span class="pre">exec_prefix</span></em></code>.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONPATH">
<code class="descname">PYTHONPATH</code><a class="headerlink" href="#envvar-PYTHONPATH" title="Permalink to this definition">¶</a></dt>
<dd><p>Augment the default search path for module files.  The format is the same as
the shell’s <span class="target" id="index-18"></span><code class="xref std std-envvar docutils literal"><span class="pre">PATH</span></code>: one or more directory pathnames separated by
<a class="reference internal" href="../library/os.html#os.pathsep" title="os.pathsep"><code class="xref py py-data docutils literal"><span class="pre">os.pathsep</span></code></a> (e.g. colons on Unix or semicolons on Windows).
Non-existent directories are silently ignored.</p>
<p>In addition to normal directories, individual <span class="target" id="index-19"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a> entries
may refer to zipfiles containing pure Python modules (in either source or
compiled form). Extension modules cannot be imported from zipfiles.</p>
<p>The default search path is installation dependent, but generally begins with
<code class="file docutils literal"><em><span class="pre">prefix</span></em><span class="pre">/lib/python</span><em><span class="pre">version</span></em></code> (see <span class="target" id="index-20"></span><a class="reference internal" href="#envvar-PYTHONHOME"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHOME</span></code></a> above).  It
is <em>always</em> appended to <span class="target" id="index-21"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a>.</p>
<p>An additional directory will be inserted in the search path in front of
<span class="target" id="index-22"></span><a class="reference internal" href="#envvar-PYTHONPATH"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONPATH</span></code></a> as described above under
<a class="reference internal" href="#using-on-interface-options"><span class="std std-ref">Interface options</span></a>. The search path can be manipulated from
within a Python program as the variable <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONSTARTUP">
<code class="descname">PYTHONSTARTUP</code><a class="headerlink" href="#envvar-PYTHONSTARTUP" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is the name of a readable file, the Python commands in that file are
executed before the first prompt is displayed in interactive mode.  The file
is executed in the same namespace where interactive commands are executed so
that objects defined or imported in it can be used without qualification in
the interactive session.  You can also change the prompts <a class="reference internal" href="../library/sys.html#sys.ps1" title="sys.ps1"><code class="xref py py-data docutils literal"><span class="pre">sys.ps1</span></code></a> and
<a class="reference internal" href="../library/sys.html#sys.ps2" title="sys.ps2"><code class="xref py py-data docutils literal"><span class="pre">sys.ps2</span></code></a> and the hook <a class="reference internal" href="../library/sys.html#sys.__interactivehook__" title="sys.__interactivehook__"><code class="xref py py-data docutils literal"><span class="pre">sys.__interactivehook__</span></code></a> in this file.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONOPTIMIZE">
<code class="descname">PYTHONOPTIMIZE</code><a class="headerlink" href="#envvar-PYTHONOPTIMIZE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal"><span class="pre">-O</span></code></a> option.  If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#cmdoption-o"><code class="xref std std-option docutils literal"><span class="pre">-O</span></code></a> multiple times.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONDEBUG">
<code class="descname">PYTHONDEBUG</code><a class="headerlink" href="#envvar-PYTHONDEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-d"><code class="xref std std-option docutils literal"><span class="pre">-d</span></code></a> option.  If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#cmdoption-d"><code class="xref std std-option docutils literal"><span class="pre">-d</span></code></a> multiple times.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONINSPECT">
<code class="descname">PYTHONINSPECT</code><a class="headerlink" href="#envvar-PYTHONINSPECT" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-i"><code class="xref std std-option docutils literal"><span class="pre">-i</span></code></a> option.</p>
<p>This variable can also be modified by Python code using <a class="reference internal" href="../library/os.html#os.environ" title="os.environ"><code class="xref py py-data docutils literal"><span class="pre">os.environ</span></code></a>
to force inspect mode on program termination.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONUNBUFFERED">
<code class="descname">PYTHONUNBUFFERED</code><a class="headerlink" href="#envvar-PYTHONUNBUFFERED" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#cmdoption-u"><code class="xref std std-option docutils literal"><span class="pre">-u</span></code></a> option.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONVERBOSE">
<code class="descname">PYTHONVERBOSE</code><a class="headerlink" href="#envvar-PYTHONVERBOSE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string it is equivalent to specifying the
<a class="reference internal" href="#id4"><code class="xref std std-option docutils literal"><span class="pre">-v</span></code></a> option.  If set to an integer, it is equivalent to specifying
<a class="reference internal" href="#id4"><code class="xref std std-option docutils literal"><span class="pre">-v</span></code></a> multiple times.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONCASEOK">
<code class="descname">PYTHONCASEOK</code><a class="headerlink" href="#envvar-PYTHONCASEOK" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set, Python ignores case in <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal"><span class="pre">import</span></code></a> statements.  This
only works on Windows and OS X.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONDONTWRITEBYTECODE">
<code class="descname">PYTHONDONTWRITEBYTECODE</code><a class="headerlink" href="#envvar-PYTHONDONTWRITEBYTECODE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set to a non-empty string, Python won’t try to write <code class="docutils literal"><span class="pre">.pyc</span></code>
files on the import of source modules.  This is equivalent to
specifying the <a class="reference internal" href="#id1"><code class="xref std std-option docutils literal"><span class="pre">-B</span></code></a> option.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONHASHSEED">
<code class="descname">PYTHONHASHSEED</code><a class="headerlink" href="#envvar-PYTHONHASHSEED" title="Permalink to this definition">¶</a></dt>
<dd><p>If this variable is not set or set to <code class="docutils literal"><span class="pre">random</span></code>, a random value is used
to seed the hashes of str, bytes and datetime objects.</p>
<p>If <span class="target" id="index-23"></span><a class="reference internal" href="#envvar-PYTHONHASHSEED"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONHASHSEED</span></code></a> is set to an integer value, it is used as a fixed
seed for generating the hash() of the types covered by the hash
randomization.</p>
<p>Its purpose is to allow repeatable hashing, such as for selftests for the
interpreter itself, or to allow a cluster of python processes to share hash
values.</p>
<p>The integer must be a decimal number in the range [0,4294967295].  Specifying
the value 0 will disable hash randomization.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.2.3.</span></p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONIOENCODING">
<code class="descname">PYTHONIOENCODING</code><a class="headerlink" href="#envvar-PYTHONIOENCODING" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set before running the interpreter, it overrides the encoding used
for stdin/stdout/stderr, in the syntax <code class="docutils literal"><span class="pre">encodingname:errorhandler</span></code>.  Both
the <code class="docutils literal"><span class="pre">encodingname</span></code> and the <code class="docutils literal"><span class="pre">:errorhandler</span></code> parts are optional and have
the same meaning as in <a class="reference internal" href="../library/stdtypes.html#str.encode" title="str.encode"><code class="xref py py-func docutils literal"><span class="pre">str.encode()</span></code></a>.</p>
<p>For stderr, the <code class="docutils literal"><span class="pre">:errorhandler</span></code> part is ignored; the handler will always be
<code class="docutils literal"><span class="pre">'backslashreplace'</span></code>.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.4: </span>The <code class="docutils literal"><span class="pre">encodingname</span></code> part is now optional.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.6: </span>On Windows, the encoding specified by this variable is ignored for interactive
console buffers unless <span class="target" id="index-24"></span><a class="reference internal" href="#envvar-PYTHONLEGACYWINDOWSSTDIO"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONLEGACYWINDOWSSTDIO</span></code></a> is also specified.
Files and pipes redirected through the standard streams are not affected.</p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONNOUSERSITE">
<code class="descname">PYTHONNOUSERSITE</code><a class="headerlink" href="#envvar-PYTHONNOUSERSITE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this is set, Python won’t add the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a> to <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal"><span class="pre">sys.path</span></code></a>.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><span class="target" id="index-25"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONUSERBASE">
<code class="descname">PYTHONUSERBASE</code><a class="headerlink" href="#envvar-PYTHONUSERBASE" title="Permalink to this definition">¶</a></dt>
<dd><p>Defines the <a class="reference internal" href="../library/site.html#site.USER_BASE" title="site.USER_BASE"><code class="xref py py-data docutils literal"><span class="pre">user</span> <span class="pre">base</span> <span class="pre">directory</span></code></a>, which is used to
compute the path of the <a class="reference internal" href="../library/site.html#site.USER_SITE" title="site.USER_SITE"><code class="xref py py-data docutils literal"><span class="pre">user</span> <span class="pre">site-packages</span> <span class="pre">directory</span></code></a>
and <a class="reference internal" href="../install/index.html#inst-alt-install-user"><span class="std std-ref">Distutils installation paths</span></a> for
<code class="docutils literal"><span class="pre">python</span> <span class="pre">setup.py</span> <span class="pre">install</span> <span class="pre">--user</span></code>.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><span class="target" id="index-26"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0370"><strong>PEP 370</strong></a> – Per user site-packages directory</p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONEXECUTABLE">
<code class="descname">PYTHONEXECUTABLE</code><a class="headerlink" href="#envvar-PYTHONEXECUTABLE" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set, <code class="docutils literal"><span class="pre">sys.argv[0]</span></code> will be set to its
value instead of the value got through the C runtime.  Only works on
Mac OS X.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONWARNINGS">
<code class="descname">PYTHONWARNINGS</code><a class="headerlink" href="#envvar-PYTHONWARNINGS" title="Permalink to this definition">¶</a></dt>
<dd><p>This is equivalent to the <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal"><span class="pre">-W</span></code></a> option. If set to a comma
separated string, it is equivalent to specifying <a class="reference internal" href="#cmdoption-w"><code class="xref std std-option docutils literal"><span class="pre">-W</span></code></a> multiple
times.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONFAULTHANDLER">
<code class="descname">PYTHONFAULTHANDLER</code><a class="headerlink" href="#envvar-PYTHONFAULTHANDLER" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string,
<a class="reference internal" href="../library/faulthandler.html#faulthandler.enable" title="faulthandler.enable"><code class="xref py py-func docutils literal"><span class="pre">faulthandler.enable()</span></code></a> is called at startup: install a handler for
<code class="xref py py-const docutils literal"><span class="pre">SIGSEGV</span></code>, <code class="xref py py-const docutils literal"><span class="pre">SIGFPE</span></code>, <code class="xref py py-const docutils literal"><span class="pre">SIGABRT</span></code>, <code class="xref py py-const docutils literal"><span class="pre">SIGBUS</span></code> and
<code class="xref py py-const docutils literal"><span class="pre">SIGILL</span></code> signals to dump the Python traceback.  This is equivalent to
<a class="reference internal" href="#id5"><code class="xref std std-option docutils literal"><span class="pre">-X</span></code></a> <code class="docutils literal"><span class="pre">faulthandler</span></code> option.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.3.</span></p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONTRACEMALLOC">
<code class="descname">PYTHONTRACEMALLOC</code><a class="headerlink" href="#envvar-PYTHONTRACEMALLOC" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, start tracing
Python memory allocations using the <a class="reference internal" href="../library/tracemalloc.html#module-tracemalloc" title="tracemalloc: Trace memory allocations."><code class="xref py py-mod docutils literal"><span class="pre">tracemalloc</span></code></a> module. The value of
the variable is the maximum number of frames stored in a traceback of a
trace. For example, <code class="docutils literal"><span class="pre">PYTHONTRACEMALLOC=1</span></code> stores only the most recent
frame. See the <a class="reference internal" href="../library/tracemalloc.html#tracemalloc.start" title="tracemalloc.start"><code class="xref py py-func docutils literal"><span class="pre">tracemalloc.start()</span></code></a> for more information.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONASYNCIODEBUG">
<code class="descname">PYTHONASYNCIODEBUG</code><a class="headerlink" href="#envvar-PYTHONASYNCIODEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If this environment variable is set to a non-empty string, enable the
<a class="reference internal" href="../library/asyncio-dev.html#asyncio-debug-mode"><span class="std std-ref">debug mode</span></a> of the <a class="reference internal" href="../library/asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O, event loop, coroutines and tasks."><code class="xref py py-mod docutils literal"><span class="pre">asyncio</span></code></a> module.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.4.</span></p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONMALLOC">
<code class="descname">PYTHONMALLOC</code><a class="headerlink" href="#envvar-PYTHONMALLOC" title="Permalink to this definition">¶</a></dt>
<dd><p>Set the Python memory allocators and/or install debug hooks.</p>
<p>Set the family of memory allocators used by Python:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">malloc</span></code>: use the <code class="xref c c-func docutils literal"><span class="pre">malloc()</span></code> function of the C library
for all domains (<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_RAW" title="PYMEM_DOMAIN_RAW"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_RAW</span></code></a>, <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a>,
<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a>).</li>
<li><code class="docutils literal"><span class="pre">pymalloc</span></code>: use the <a class="reference internal" href="../c-api/memory.html#pymalloc"><span class="std std-ref">pymalloc allocator</span></a> for
<a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_MEM" title="PYMEM_DOMAIN_MEM"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_MEM</span></code></a> and <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_OBJ" title="PYMEM_DOMAIN_OBJ"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_OBJ</span></code></a> domains and use
the <code class="xref c c-func docutils literal"><span class="pre">malloc()</span></code> function for the <a class="reference internal" href="../c-api/memory.html#c.PYMEM_DOMAIN_RAW" title="PYMEM_DOMAIN_RAW"><code class="xref c c-data docutils literal"><span class="pre">PYMEM_DOMAIN_RAW</span></code></a> domain.</li>
</ul>
<p>Install debug hooks:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">debug</span></code>: install debug hooks on top of the default memory allocator</li>
<li><code class="docutils literal"><span class="pre">malloc_debug</span></code>: same as <code class="docutils literal"><span class="pre">malloc</span></code> but also install debug hooks</li>
<li><code class="docutils literal"><span class="pre">pymalloc_debug</span></code>: same as <code class="docutils literal"><span class="pre">pymalloc</span></code> but also install debug hooks</li>
</ul>
<p>When Python is compiled in release mode, the default is <code class="docutils literal"><span class="pre">pymalloc</span></code>. When
compiled in debug mode, the default is <code class="docutils literal"><span class="pre">pymalloc_debug</span></code> and the debug hooks
are used automatically.</p>
<p>If Python is configured without <code class="docutils literal"><span class="pre">pymalloc</span></code> support, <code class="docutils literal"><span class="pre">pymalloc</span></code> and
<code class="docutils literal"><span class="pre">pymalloc_debug</span></code> are not available, the default is <code class="docutils literal"><span class="pre">malloc</span></code> in release
mode and <code class="docutils literal"><span class="pre">malloc_debug</span></code> in debug mode.</p>
<p>See the <a class="reference internal" href="../c-api/memory.html#c.PyMem_SetupDebugHooks" title="PyMem_SetupDebugHooks"><code class="xref c c-func docutils literal"><span class="pre">PyMem_SetupDebugHooks()</span></code></a> function for debug hooks on Python
memory allocators.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.6.</span></p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONMALLOCSTATS">
<code class="descname">PYTHONMALLOCSTATS</code><a class="headerlink" href="#envvar-PYTHONMALLOCSTATS" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, Python will print statistics of the
<a class="reference internal" href="../c-api/memory.html#pymalloc"><span class="std std-ref">pymalloc memory allocator</span></a> every time a new pymalloc object
arena is created, and on shutdown.</p>
<p>This variable is ignored if the <span class="target" id="index-27"></span><a class="reference internal" href="#envvar-PYTHONMALLOC"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONMALLOC</span></code></a> environment variable
is used to force the <code class="xref c c-func docutils literal"><span class="pre">malloc()</span></code> allocator of the C library, or if
Python is configured without <code class="docutils literal"><span class="pre">pymalloc</span></code> support.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.6: </span>This variable can now also be used on Python compiled in release mode.
It now has no effect if set to an empty string.</p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONLEGACYWINDOWSFSENCODING">
<code class="descname">PYTHONLEGACYWINDOWSFSENCODING</code><a class="headerlink" href="#envvar-PYTHONLEGACYWINDOWSFSENCODING" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, the default filesystem encoding and errors mode
will revert to their pre-3.6 values of ‘mbcs’ and ‘replace’, respectively.
Otherwise, the new defaults ‘utf-8’ and ‘surrogatepass’ are used.</p>
<p>This may also be enabled at runtime with
<a class="reference internal" href="../library/sys.html#sys._enablelegacywindowsfsencoding" title="sys._enablelegacywindowsfsencoding"><code class="xref py py-func docutils literal"><span class="pre">sys._enablelegacywindowsfsencoding()</span></code></a>.</p>
<p>Availability: Windows</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.6: </span>See <span class="target" id="index-28"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0529"><strong>PEP 529</strong></a> for more details.</p>
</div>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONLEGACYWINDOWSSTDIO">
<code class="descname">PYTHONLEGACYWINDOWSSTDIO</code><a class="headerlink" href="#envvar-PYTHONLEGACYWINDOWSSTDIO" title="Permalink to this definition">¶</a></dt>
<dd><p>If set to a non-empty string, does not use the new console reader and
writer. This means that Unicode characters will be encoded according to
the active console code page, rather than using utf-8.</p>
<p>This variable is ignored if the standard streams are redirected (to files
or pipes) rather than referring to console buffers.</p>
<p>Availability: Windows</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 3.6.</span></p>
</div>
</dd></dl>

<div class="section" id="debug-mode-variables">
<h3>1.2.1. Debug-mode variables<a class="headerlink" href="#debug-mode-variables" title="Permalink to this headline">¶</a></h3>
<p>Setting these variables only has an effect in a debug build of Python, that is,
if Python was configured with the <code class="docutils literal"><span class="pre">--with-pydebug</span></code> build option.</p>
<dl class="envvar">
<dt id="envvar-PYTHONTHREADDEBUG">
<code class="descname">PYTHONTHREADDEBUG</code><a class="headerlink" href="#envvar-PYTHONTHREADDEBUG" title="Permalink to this definition">¶</a></dt>
<dd><p>If set, Python will print threading debug info.</p>
</dd></dl>

<dl class="envvar">
<dt id="envvar-PYTHONDUMPREFS">
<code class="descname">PYTHONDUMPREFS</code><a class="headerlink" href="#envvar-PYTHONDUMPREFS" title="Permalink to this definition">¶</a></dt>
<dd><p>If set, Python will dump objects and reference counts still alive after
shutting down the interpreter.</p>
</dd></dl>

</div>
</div>
</div>


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  <h3><a href="../contents.html">Table Of Contents</a></h3>
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<li><a class="reference internal" href="#">1. Command line and environment</a><ul>
<li><a class="reference internal" href="#command-line">1.1. Command line</a><ul>
<li><a class="reference internal" href="#interface-options">1.1.1. Interface options</a></li>
<li><a class="reference internal" href="#generic-options">1.1.2. Generic options</a></li>
<li><a class="reference internal" href="#miscellaneous-options">1.1.3. Miscellaneous options</a></li>
<li><a class="reference internal" href="#options-you-shouldn-t-use">1.1.4. Options you shouldn’t use</a></li>
</ul>
</li>
<li><a class="reference internal" href="#environment-variables">1.2. Environment variables</a><ul>
<li><a class="reference internal" href="#debug-mode-variables">1.2.1. Debug-mode variables</a></li>
</ul>
</li>
</ul>
</li>
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